This invention relates to apparatus for roasting or calcining fine grained material such as cement raw meal, limestone or dolomite and has particular application in a cement producing system utilizing a suspension-type preheater, a stationary calcining furnace and a separate clinkering furnace followed by a cooler.
The present invention is an improvement over U.S. Pat. No. 4,381,916 issued May 3, 1983. In that patent, it is disclosed that it is desirable in an ore roasting apparatus similar to the present invention to recirculate material to be roasted or calcined through the calcining furnace of the apparatus. In that patent there is disclosed a suspension-type preheater followed by a separate calcining furnace followed by a clinkering furnace and a cooler. Cement raw meal or other material to be roasted is preheated in the preheater, then supplied to the calcining furnace. Material discharged from the calcining furnace is supplied to a separate processer such as the clinkering furnace while a portion of it is recirculated back to the calcining furnace for further calcining. The advantage of such a system is that the fine material to be calcined or roasted is exposed to the temperature in the calciner for a greater period of time so that a higher percentage of material is calcined at a given temperature.
According to the present invention, a practical apparatus has been provided for carrying out the process disclosed in the aforementioned U.S. patent.
In cement clinker producing plants and in other thermal processing installations, large pieces of material such as pieces of broken refractory, tramp iron and the like can work its way through a preheater to plug downstream apparatus. These large chunks of material should be separated from the system or they will plug the recirculation system. It is best if these oversized particles can be supplied directly to the kiln.
It is also known that in material roasting systems such as those to which the present invention relates that due to the sticky nature of the intermediate material, plugging of the system can occur and it is necessary to provide a by-pass system around the recirculation system in the event of such plugging.